1. Field of the Invention
The present invention relates to a radiation image detecting device having a dose detection sensor for performing exposure control of a radiographic image, and a radiation imaging system using the radiation image detecting device.
2. Description Related to the Prior Art
In a medical field, a radiation imaging system, for example, an X-ray imaging system using X-rays as a kind of radiation is known. The X-ray imaging system is constituted of an X-ray generating device for producing the X-rays, and an X-ray imaging device for taking an X-ray image of an object by receiving the X-rays passed through the object (patient). The X-ray generating device has an X-ray source for emitting the X-rays to the object, a source control device for controlling the operation of the X-ray source, and an emission switch for inputting to the source control device a command to operate the X-ray source. The X-ray imaging device has an X-ray image detecting device for detecting the X-ray image based on the X-rays passed through the object, and a console, for controlling the operation of the X-ray image detecting device and saving and displaying the X-ray image.
As the X-ray image detecting device, a flat panel detector (FPD) for detecting the X-ray image as an electric signal has come into widespread use. The FPD includes a detection panel having an imaging surface having a matrix of pixels for accumulating electric charge in accordance with an X-ray dose incident thereon and circuitry for operating the detection panel. The detection panel accumulates the signal charge on a pixel-by-pixel basis and converts the accumulated electric charge into a voltage signal by a signal processing circuit, to detect the X-ray image of the object and output the X-ray image as digital image data. Also there is a portable type X-ray image detecting device called electronic cassette, which contains the FPD in a cassette housing.
In the X-ray imaging, there are cases where a scattered radiation removing member called grid is used for the purpose of reducing the influence of scattered radiation that is produced by the X-rays in passing through the object. The grid is disposed between the object and the X-ray image detecting device. The grid is composed of X-ray absorbing portions and X-ray transmitting portions arranged alternately. Each X-ray absorbing portion is made of an X-ray absorbing and opaque material such as lead into the shape of a slender strip. Each X-ray transmitting portion is made of an X-ray transparent material such as aluminum into the shape of a slender strip. Since the X-ray absorbing portions and the X-ray transmitting portions are arranged alternately in one direction, the X-ray absorbing portions and the X-ray transmitting portions form a stripe pattern. Such a grid is disposed between the imaging surface of the detection panel and the object. The use of the grid facilitates obtaining a high-contrast image with reduced influence of the scattered radiation, because most of the scattered radiation is absorbed by the X-ray absorbing portions in the grid before reaching the imaging surface. The grid is attached to an imaging stand or a housing of the X-ray image detecting device in use.
One of items representing the type of the grid is a grid density that represents the number of the X-ray absorbing portions per unit width. There are various grid densities within the confines of 26/cm 100/cm, for example. Taking the case of a grid density of 40/cm (4/mm) as an example, a grid pitch, being the sum of the width of a pair of X-ray absorbing portion and X-ray transmitting portion is 250 μm.
Some X-ray image detecting devices have an automatic exposure control (AEC) function, which stops an X-ray emission from the X-ray source at the instant when an X-ray dose emitted from the X-ray source has reached a predetermined emission stop threshold value, in order to perform exposure control of the X-ray image (refer to U.S. Pat. No. 6,952,465 corresponding to Japanese Patent Laid-Open Publication No. 2004-167075 and U.S. Pat. No. 6,944,266 corresponding to Japanese Patent Laid-Open Publication No. 2004-166724, for example). Such X-ray image detecting devices have dose detection sensors, which detect an X-ray dose passed through the object and output a signal corresponding to the detected dose.
The U.S. Pat. No. 6,952,465 describes an X-ray image detecting device that is provided with stripe-shaped dose detection sensors having a length of 500 pixels in an imaging surface of a detection panel, besides pixels. According to the U.S. Pat. No. 6,952,465, the dose detection sensors are disposed such that a longitudinal direction (stripe direction) of the striped dose detection sensors is not in parallel with (for example, orthogonal to) a stripe direction of a grid. Accordingly, even if misalignment occurs in geometrical disposition being the positional relation between the grid and the dose detection sensors, stable AEC is performed by reducing variation in an output value of a signal outputted from the dose detection sensor.
In other words, since the dose detection sensors are disposed in the imaging surface, the misalignment occurs in the geometrical disposition between the grid and the dose detection sensors owing to an attachment backlash of the grid, a manufacturing error of the grid, and the like. Each of X-ray absorbing portions and X-ray transmitting portions of the grid has a width of the order of micrometers. Thus, the attachment backlash or the manufacturing error of the grid easily causes the misalignment of the order of one X-ray absorbing portion or one X-ray transmitting portion between the grid and the dose detection sensors. The misalignment in the geometrical disposition between the grid and the dose detection sensors causes variation in the amount of X-rays incident upon the dose detection sensors, even if an X-ray emission amount is the same, and hence results in variation in the output value of the dose detection sensor. A variation range of the output value of the dose detection sensor is maximized in a case where the stripe direction of the dose detection sensors is in parallel with the stripe direction of the grid.
For example, in a case where the stripe direction of the dose detection sensors is in parallel with the stripe direction of the grid, the striped dose detection sensors may be hidden behind the X-ray absorbing portions throughout its longitudinal direction, or contrarily situated behind the X-ray transmitting portions. In a case where the entirety of the dose detection sensor is hidden behind the X-ray absorbing portion, the X-ray incident amount is reduced throughout the dose detection sensor, and hence the output value of the dose detection sensor is minimized. On the contrary, even if the X-ray emission amount is the same, in a case where the entirety of the dose detection sensor is situated behind the X-ray transmitting portion, the X-ray incident amount is increased throughout the dose detection sensor, and hence the output value of the dose detection sensor is maximized. As described above, paralleling the stripe direction of the dose detection sensors to the stripe direction of the grid increases the variation range of the output value of the dose detection sensor caused by the misalignment in the geometrical disposition between the grid and the dose detection sensors.
Thus, according to the U.S. Pat. No. 6,952,465, the striped dose detection sensors are disposed not in parallel with the stripe direction of the grid, so that a part of the dose detection sensor is always disposed behind the X-ray absorbing portion and another part of the dose detection sensor is always disposed behind the X-ray transmitting portion, even if the geometrical disposition between the grid and the dose detection sensors is misaligned. Thereby, the X-ray incident amount is relatively low at a part of the dose detection sensor, while relatively high at another part of the detection sensor, so the output value of the dose detection sensor is leveled. Therefore, as compared with the case of paralleling the stripe direction of the dose detection sensors to the stripe direction of the grid, it is possible to prevent the variation in the output value of the dose detection sensor caused by the misalignment in the geometrical disposition between the grid and the dose detection sensors, and carry out the stable AEC.
In an embodiment of the U.S. Pat. No. 6,952,465, a pixel is of a size of 105 μm×105 μm. The dose detection sensor has a size of 500 pixels, and hence a length of the order of 105 μm×500=52500 μm (approximately 50 mm). The dose detection sensor is substituted for the pixels, or disposed in space between the adjoining pixels. In the case of disposing the dose detection sensor in the space between the pixels, the pixels adjoining to the dose detection sensor are downsized, to secure space for the dose detection sensor. A plurality of the dose detection sensors is disposed in a predetermined area.
Also, the U.S. Pat. No. 6,944,266 describes an X-ray image detecting device in which detection pixels (referred to as AEC pixels in the U.S. Pat. No. 6,944,266) functioning as dose detection sensors are substituted as some of pixels, instead of providing the striped dose detection sensors. The U.S. Pat. No. 6,944,266 uses so-called non-destructive readout pixels from which an output value is read out while the pixels keep holding accumulated electric charge, and the detection pixels are also read out in a non-destructive manner.
In the U.S. Pat. No. 6,944,266, since the detection pixels are disposed in an imaging surface, misalignment in the geometrical disposition between a grid and the detection pixels causes variation in an output value of the detection pixel, just as with the U.S. Pat. No. 6,952,465. The U.S. Pat. No. 6,944,266 deals with the problem of variation in the output value of each detection pixel by calibration, which calibrates the output value of each detection pixel.
To be more specific, according to the U.S. Pat. No. 6,944,266, the X-rays are evenly applied to the imaging surface having the grid attached, to obtain a gain image representing the output value of each detection pixel in the imaging surface. In the gain image, variation in the output values of the detection pixels in a state of attaching the grid is reflected. In AEC of actual imaging, the output value of each detection pixel is calibrated with the gain image to correct the output value of each detection pixel. The output value of each detection pixel varies with not only the type of the grid specifying the grid density, but also an imaging condition including an X-ray dose and X-ray quality depending on a tube voltage. Even if the type of the grid and the imaging condition are the same, an attachment backlash of the grid or a manufacturing error causes misalignment in the geometrical disposition between the grid and the detection pixels, so that the gain image is obtained whenever imaging is carried out.
In the U.S. Pat. No. 6,952,465, the space for the striped dose detection sensors is secured by substituting or downsizing the pixels, so the obtained X-ray image has low density at portions corresponding to the dose detection sensors. The difference in density is large between the portion corresponding to the dose detection sensor and other portions adjoining thereto, and hence manifests itself as a strip of density step. Since the striped dose detection sensors have a length of approximately 50 mm, being a size visible to a human eye, the density step of the X-ray image is also of a size visible to the human eye and very conspicuous. To eliminate such a density step, the U.S. Pat. No. 6,952,465 discloses performing a defect correction with regarding the dose detection sensors as defect pixels, but a defect correction requires an effort at preparing correction data. Furthermore, the dose detection sensors are large, being approximately 50 mm. Thus, it is difficult to completely eliminate the defect by the defect correction to the extent of being invisible, and it is feared that the image quality of the X-ray image may be degraded.
In the case of the U.S. Pat. No. 6,944,266, some of the pixels are used as the detection pixels and the output values of the detection pixels are read out in a non-destructive manner, so no density step occurs in the X-ray image, in contrast to the U.S. Pat. No. 6,952,465. Therefore, there is no problem of efforts at the defect correction and no problem of degradation in the image quality of the X-ray image. However, in the case of the X-ray image detecting device described in the U.S. Pat. No. 6,944,266, the gain image has to be obtained whenever imaging is carried out, and hence there is another problem that obtaining the gain image requires time and effort. Furthermore, if the geometrical disposition between the grid and the detection pixels is misaligned after obtainment of the gain image and before carrying out actual imaging, the correction cannot be performed appropriately with the obtained gain image.